1. Field of the Invention
This invention relates to display systems. Specifically, this invention relates to systems which display alpha-numeric data in addition to video data such as that obtained from a radar system.
While the preferred embodiment will be described herein with reference to a particular application, it is to be understood that the invention is not limited thereto. Those of ordinary skill in the art and access to the teachings of this invention will recognize other applications within the scope thereof.
2. Description of the Prior Art
The desirability of displaying alpha-numeric characters in conjunction with radar and/or sonar returns is now widely recognized. For example, many air traffic control centers are now equipped with radar consoles which display such information as aircraft indentification, speed and altitude in proximity to the radar video signal. Such radar consoles typically generate the video image utilizing a random scan.
A random scan is one which the electron beam is deflected to draw each image individually on a phosphorescent screen. When this technique is used to display alpha-numeric data with radar video the images may begin to fade before all the information can be written. As a result, there is a limitation on the amount of data that can be displayed within a time frame determined in part by the video retention period of the display. For this reason, a raster scan such as that employed with conventional television systems is being considered as an alternative.
Raster scan provides a significant improvement over random scan in allowing the video image to be stored in memory which may be electronically refreshed periodically to prevent fading. Raster scan display of alpha-numerics with radar video typically requires two memories, often referred to as bit map memories. One for the video and one for the graphics. Each memory holds the information to be displayed until it is written on the screen line by line by the electron beam.
A raster scan system typically requires two digital-to-analog convertors to interface each digital memory with the display screen. As an alternative, the memories may be multiplexed through a single D/A convertor. In either case, prior art systems for raster scan display of video with alpha-numerics requires the synchronization of the two memories to ensure the display of alpha-numerics in proper spatial relation to video. Synchronization electronics adds to the cost, complexity and maintenance problems of raster scan systems. Thus, it has been found desirable to reduce the cost and complexity associated with such systems.